Passing off is the oldest theory of

Passing off is the oldest common-law doctrine of unfair competition: it stops one trader from misrepresenting his goods or business as those of another and so taking the goodwill the other has built up. It grew out of the wider law against unfair competition between traders, which is the established name for this field; 'unfair trade', 'unfair practice' and 'unfair business' are not the recognised doctrine.